Read the jeans before adding anything
Straight jeans can be narrow through the thigh or generous from hip to hem. They may sit at the waist, below it or somewhere between. Those differences matter more to an outfit than the word straight on the label. Put them on with a plain top and look at where the waistband and hem actually finish.
A stiff, dark pair often makes the rest of an outfit appear more structured. Faded denim can make the same jacket seem easier. If a look feels heavier than you intended, changing a dark boot for a lighter shoe may do more than changing the top. You are working with areas of colour as well as the shapes of the garments.
Decide where to show the waist
A short knit that reaches the waistband leaves the line of the jeans visible. A long shirt conceals it and creates a longer block of colour above the legs. Both can work; they simply give different results. Try one small front tuck before deciding that the entire top needs replacing.
Watch the fabric at the hip. A thick sweatshirt tucked into close-fitting denim can make a ridge that feels uncomfortable, particularly while sitting. An untucked sweatshirt with a clear hem may be better. Conversely, a soft shirt can sometimes sit inside the waistband without adding much bulk. There is no universal requirement to tuck a top into straight jeans.
See the shoe in motion
The hem should let you walk without catching it under your heel. Beyond that, choose its relationship with the shoe. A little ankle above a loafer feels different from denim resting over the top of a boot. Test the combination with the actual footwear, because two pairs described as flat can have very different soles and uppers.
Cuffing is a reversible way to explore length. One wide turn introduces a visible band; several narrow rolls can become bulky. If you continually fold the same pair to the same point, a professional alteration may be worth considering, but decide only after trying the shoes you wear most often.
Give the outfit one direction
For a relaxed lunch, a soft cardigan and familiar trainers may be enough. For a meeting in an informal workplace, add a shirt and a jacket whose shoulders fit comfortably. For dinner, a draped top or a different pair of earrings can change the emphasis while the jeans stay exactly as they are.
Photograph two versions in similar light if you find it hard to see the difference. Notice the overall proportions rather than searching for a supposedly correct body shape. The useful question is which combination feels like the day you are about to have.
